1. Field of the Invention
The present invention relates to a method of manufacturing semiconductor devices, and more particularly, to a method of forming a material layer during the manufacture of semiconductor devices using the technique of atomic layer deposition (ALD) and a method of forming the dielectric layer of a capacitor of a semiconductor device using ALD.
2. Description of the Related Art
The decrease in cell capacitance resulting from a reduction in available memory cell area makes it difficult to increase the integration density of semiconductor memory devices. Decreased cell capacitance degrades the data readability from memory cells, increases the soft error rate and also hinders low-voltage operation of the semiconductor memory devices.
Several techniques have been utilized to maintain or increase the cell capacitance without significantly increasing the wafer area occupied by the memory cell. Two such methods of increasing capacitance within a limited cell area including thinning the dielectric layer of the capacitor and/or increasing the effective surface area of the capacitor lower electrode using a cylindrical or pin-shaped structure. However, for memory devices such as dynamic random access memories (DRAMs) having capacities of 1 gigabit or more, it is difficult to obtain sufficient capacitance to ensure satisfactory operation of the memory devices with either of the methods noted above.
To further increase the capacitance that can be achieved within a given memory cell size, research efforts have been directed toward the use of materials having a higher dielectric constant (κ), such as Ta2O5, Y2O3, HfO2, ZrO2, Nb2O5, BaTiO3 or SrTiO3, as the dielectric layer of a capacitor is being actively conducted.
Conventionally, a Ta2O5 film has been widely used because of its relatively high dielectric constant and high thermal stability. However, the use of the Ta2O5 film has a potential problem in that it typically is highly reactive with a polysilicon film. If the lower electrode of capacitor is formed from polysilicon, oxygen (O) from the Ta2O5 film can react with silicon of the polysilicon layer during the formation of the Ta2O5 film or during a subsequent thermal treatment after the Ta2O5 film has been formed to from silicon dioxide at the surface of the polysilicon layer. Further, oxygen vacancies within Ta2O5 film may increase leakage current.
In an attempt to address this problem, lower electrodes have been formed from materials that are believed to be relatively more difficult to oxidize than polysilicon. Examples of such materials include noble metals such as platinum (Pt), ruthenium (Ru) and iridium (Ir) or conductive metal nitride films such as titanium nitride (TiN). However, the use of a noble metal or a metal nitride presents other potential problems.
A conventional tantalum oxide (Ta2O5) film is typically formed by chemical vapor deposition (CVD) in an oxygen atmosphere using pentaethoxide tantalum (PET), Ta(OCH3)5 or TaCl5 as a tantalum source gas and oxygen (O2), water (H2O), hydrogen peroxide (H2O2) or nitrous oxide (N2O) as an oxygen source gas. Notwithstanding any advantages associated therewith, a composition of these source gases often negatively impacts the coverage of the Ta2O5 film, presumably due to the oxidation of the lower electrode. For example, if ruthenium (Ru) is used as a lower electrode, the surface of the Ru layer can be oxidized to form a RuO2 film that minimizes or prevents the formation of the desired Ta2O5 film. This problem often occurs when a Ta2O5 film is used as a dielectric layer in a cylindrical or concave-shaped capacitor having a large aspect ratio. In such an instance, the Ta2O5 film will tend to be more thinly deposited or largely missing from the portion of the Ru electrode in the lower portion of a cylindrical opening, while the Ta2O5 film is thickly deposited on the upper portion of the opening, thereby resulting in poor step coverage of the resulting Ta2O5 film.
Generally, thin films such as dielectric films are formed using deposition methods such as CVD, low-pressure chemical vapor deposition (LPCVD), plasma-enhanced chemical vapor deposition (PECVD) and/or sputtering. The step coverage that is typically obtained with CVD-based methods, however, remained less then desired. Accordingly, atomic layer deposition (ALD) processes have been proposed as an alternative to CVD-based deposition methods because the ALD processes can be performed at lower temperatures while exhibiting improved step coverage.
One such ALD process technology is disclosed in U.S. Pat. No. 6,124,158, in which a first reactant is introduced to react with the treated surface to form a bonded monolayer of reactive species. A second reactant is then introduced to react with the bonded monolayer to form a thin layer of the desired material on the treated surface. After each step in the cycle, the reaction chamber is purged with an inert gas to prevent reaction except at the treated surface.
Since an ALD film has low thermal budget, excellent step coverage, and excellent thickness control and uniformity, efforts have been made to develop methods whereby a metal oxide such as Ta2O5, Y2O3, HfO2, ZrO2, or Nb2O5, may be deposited using an ALD method to form the high-κ dielectric layer of a capacitor. One such effort formed a metal oxide by ALD using as the metal precursor a halide such as HfCl4 in combination with an oxidant such as O2, H2O, H2O2 or N2O. However, efforts of forming such thin films using halide group precursors tend to result in unsatisfactory levels of step coverage.
Further, when H2O is used as the oxidant, hydrogen (H) radicals tend to react with halogen ligands separated from HfCl4 to thereby form a gas including hydrochloric acid (HCl). Because the HCl gas tends to etch the thin film on the semiconductor device, the surface morphology of the resulting thin film is compromised. In addition, the metal may combine with the —H and/or —OH groups formed, resulting in the incorporation of undesirable impurities, e.g., metal hydroxides, into the metal oxide film. If a metal oxide film containing impurities such as metal hydroxides is utilized as a dielectric layer in a semiconductor device, the metal hydroxides may act as trap sites or a current leakage sites, thereby degrading the dielectric characteristics of the resulting device.